Linking gradient ramp to slider

You would add an expression control slider to the layer, tie the y position of the Start Value for the gradient to the slider, and

then add 100 to the y value of the End Value for the gradient. If the gradient is on a shape layer, the expressions would look like this:

// Start property
t = - thisComp.height/2;
y = t + effect("Slider Control")("Slider") * 20;
[value[0], y]

// End property
x = content("Rectangle 1").content("Gradient Fill 1").startPoint[0];
y = content("Rectangle 1").content("Gradient Fill 1").startPoint[1] + 100;
[x, y]

Because the gradient position is centered on the layer, you would define the top of the comp by subtracting half the comp height. To make the slider work on a percentage of the comp height, instead of having to scroll to numbers over 1000, multiply the slider value by 20, so zero to one hundred will give you a slider that moves the start point from the top to the bottom of the comp.  

If you are using Gradient Fill, you don't have to subtract half the comp height.